Among examples of the conventionally-known engines equipped with a secondary air supply device are ones in which an exhaust pipe is provided for directing exhaust gas to an exhaust muffler and in which a secondary air introduction pipe is provided in communication with the exhaust pipe so that air (secondary air) is introduced into the exhaust muffler via the secondary air introduction pipe using exhaust pulsation of the engine. By air being introduced into the exhaust muffler via the secondary air introduction pipe like this, an oxidation-reduction reaction (detoxifying reaction) can be caused in exhaust gas by means of a catalyst (catalytic) converter provided within the exhaust muffler.
Further, one example of such engines equipped with a secondary air supply device is disclosed in Japanese Patent No. 2604659 (hereinafter referred to as “'JP '659”), in which a cylinder of the engine extends obliquely so that a space is defined beneath the cylinder, and in which an opening portion of the secondary air introduction pipe opens to the space. In such an engine, air is sucked through the opening portion into the secondary air introduction pipe by exhaust pulsation of the engine.
In the engine disclosed in 'JP '659, in which the opening portion of the secondary air introduction pipe opens to the space defined beneath the cylinder, cooling air blown from a cooling fan is directed to the space beneath the cylinder so that the engine (cylinder in particular) is cooled with the cooling air. Because the cooling air is directed to the space to which the opening portion of the secondary air introduction pipe opens as noted above, dust, dirt, rain water, etc. may undesirably enter through the opening. Thus, there arises a need to remove such dust, dirt, rain water, etc. from the air sucked in through the opening portion, which would make maintenance of a filter etc. cumbersome operation.